LCD 16x2 have around 16 pins which needs to be connected to microcontroller in order to make it work. But by using an I2C extender, we only needs to connect 2 pins (SDA and SCL) to the microcontroller.
SPI is not very different from I2C. To enable a slave device, we need to pull the CS pin low and after our read or write is complete, just pull the pin high again. This will disable the slave device.